1. Field of the Invention
The present invention relates to energy systems as used on drilling rigs. More particularly, the present invention the relates to drilling rigs that are supplied with power from a natural gas engine/generator. Additionally, the present invention relates systems for supplying power and for storing power through the use of batteries.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
In the field of oil well drilling, significant amount of power is required during the drilling activity. The power requirements, as used on a drilling rig, serve to supply the drawworks, the mud pumps, the top drives, the rotary tables, the dynamic braking systems and other peripheral loads. In oil well drilling activities, oversized power systems are often utilized so as to meet the “peak” power requirements.
Historically, the number of engines/generators that are used and are typically online are more than the required load of the application due to the redundancy and necessary peak KW and VAR demand during certain aspects of the operation. In particular, these peak demands are during the “tripping” of the pipe or drill stem.
During normal operations, there is a base load of lighting, pumps, agitators, mixers, air compressors, etc. This base load can make up typical loads of 400-600 kilowatts. The mud pumps, top drives and rotary tables contribute another fairly consistent KW demand. This demand will vary based on the particular well, depth of drilling, and material being drilled.
During oil well drilling activities, the most intermittent load is the drawworks. This intermittent load is directed toward the peak demand during the raising or lowering of the drill pipe upwardly and downwardly in the well. This peak demand can have loads as much as two to three times the base loads of the other demands on the drilling rig.
When drilling and at times when the downhole tool has to be inspected or changed, it is required to pull all of the drill pipe from the hole. This distance can be 10,000 feet or more. The drill pipe must be taken apart and stacked as it is being removed. After repair or replacement, the reverse procedure must take place so as to reinsert all the components back to the desired depth. During the tripping in or out of the hole, the driller (operator) demands extreme power consumption and very quick bursts as the driller raises (or lowers) the string of drill pipe. Since there is a limitation on the height of the drilling mast, the operator must lift the sections in increments and unscrew the different sections. These sections are stacked one at a time. This process is repeated during the reinsertion of the drill pipe back into the hole. This process is referred to as “making a trip”. The intermittent high demand occurs when this load (300,000 pounds or more) occurs over and over again. The load is inconsistent since the weight of the drill stem becomes less and less as sections are removed. The base load requirements for the drilling rig are approximately 600 to 800 KW. The peak demand can be 1.5 MW and as high as 2.0 MW. Because of these power requirements, the emissions of the engines/generators for a typical land rig are quite high. Newer engines can have much lower NOx output than earlier engines. There are also large amounts of carbon dioxide emissions. The fuel consumption during these intermittent demands can be quite significant.
Natural gas generators are being used for land-based drilling application and offer unique advantages in reduced exhaust emissions and fuel cost savings compared to more commonly used diesel engine/generators. Natural gas engine/generators make it much simpler to meet ever more stringent emissions regulations, particularly for oxides of nitrogen (NOx). Fuel cost on a consumed energy basis is significantly when compared to diesel engine generators. Additionally, natural gas engine/generators have the added advantage of accepting wellhead gas for further cost benefits.
For the typical drilling rig operations, power is derived from the contractor's onsite generators with the prime movers being either diesel engines or spark-ignited engines fueled by natural gas, or compression-ignited engines using a combination of diesel and natural gas fuels (referred to as dual fuel generators). Diesel engines have much better load characteristics compared to natural gas engines and therefore respond more reliably to changes in loads as drilling functions abruptly demand power requirements, such as tripping of the drill string.
When natural gas engine/generators utilize wellhead gas, this eliminates the need for the delivery of natural gas by trucks or by rail. It also minimizes the need to vent or flare the gas. Unfortunately, the wellhead natural gas is a variable supply. The content of the natural gas produced at the wellhead can have various components. As such, this natural gas will need to be scrubbed prior to being used by the natural gas engine/generator. As a result, the supply of wellhead gas can be of a variable supply and a variable energy content.
Dual fuel generators offer some reduced fuel costs, but this is limited since the generator must switch from high volume ratios of natural gas back to high volume ratios of diesel in order to meet the block loading and load shedding conditions forced by changing rig power demands. A gas engine generator fueled entirely by compressed natural gas, pipeline gas, or LNG, would eliminate the use of diesel. This enables gas engine generators to provide improved fuel cost savings and reduced emissions when compared to dual fuel generators.
Referring to FIG. 1, there is shown a prior energy system for use with the various loads of a drilling rig. In particular, the energy system 10 includes engines 12, 14 and 16. Engine 12 operates generator 18. Engine 14 operates generator 20. Engine 16 operates generator 22. The generators 18, 20 and 22 will pass AC power along respective lines 24, 26 and 28 to a common AC bus 30. Typically, the various engine/generators, as shown in FIG. 1, are diesel engines. However, it is possible that such engine/generator combination could be also natural gas engine/generators.
A common DC bus 32 is illustrated as connected to the various components 34, 36, 38, 40 and 42 of the drilling rig. Load 34 is a DB module. Load 36 is the drawworks. Load 38 is the top drive. Loads 40 and 42 are the mud pumps. Each of these loads 34, 36, 38, 40 and 42 are switchably connected to the common DC bus 32.
The AC bus is configured to supply power to the hotel loads 44 and 46 of the drilling rig. Hotel loads 44 and 46 can include air conditioning and heating, lighting, and other energy requirements of the drilling rig.
A first rectifier 48 is connected between the AC bus 30 and the DC bus 32. Rectifier 38 serves to convert the AC power to DC power. Similarly, the other rectifier 50 is connected between the AC bus 30 and the DC bus 32, also to convert the AC power to DC power. The DC power is properly utilized by the loads 34, 36, 38, 40 and 42.
In FIG. 1, it can be seen that there is a resistive load bank 52 that is connected, by a switch, to the AC bus. As such, any excess energy that is provided by the various engine/generator combinations can be burned as heat by the resistive load bank 52.
Currently-used gas engine/generators that are used to power a drilling rig must be controlled to accept a lower level of transient response than is possible with diesel power. This requires the estimating of the transient response capability of the gas engine/generator and the determining of how the rate of application or rate of load removal can be reduced to make the system work. Unfortunately, this results in reduced power rates and decreased rig productivity, even with the use of a ballast load or resistive load bank 52. A typical approach is to create a load profile of the rig's expected operations in terms of power required versus time. The creation of this profile for both the desired “ideal” loading rates and for the drill site's minimum requirements will establish the minimum and maximum loading conditions for the rig powerhouse. Gas engine/generator operation is then controlled within these minimum and maximum values to attempt to minimize power interruptions from forced generator failure.
FIG. 2 shows the transient response of the natural gas engine/generator during the adding of load of the shedding of load. All gensets to either have a response to such added load or shredded load. Changes in voltage and frequency associated with this transient response is dependent on the generator type (e.g., diesel compression versus natural gas spark-ignited engine) and the magnitude of the load change, where these step loads are described as some percentage of full rated power.
The transient response and steady state stability of generator set engines can vary because of a number of factors, such as engine model, engine speed, aspiration, power factor, governor and the presence of an idle circuit. Diesel engines have a short mechanical path between the governor actuator and the fuel delivery system to the combustion chamber. This system responds quickly and in a more stable manner to load change requests from the governor. Whenever a large load is added to a generator set, engine speed temporarily slows down, or dips, before returning to its steady-state condition. When a load is removed, engine speed increases, or overshoots, temporarily. Since generator frequency is determined by engine RPM, the quality of electrical power is impacted. The measurements of these temporary speed changes is referred to as “transient response”.
The transient response is measured by percentage frequency change and duration. This relationship is illustrated in FIG. 3 herein. The amount of time it takes for the engine to return to steady-state operation is referred to as “recovery time”. This can vary from as little as one second to twenty seconds. In general, the greater the load added to the bus, the greater the percentage of dip and the longer it will take the engine to recover. Dips are generally more critical than overshoots because severe block loading can stall the engine and cause generator voltage to collapse.
There no formal documentation of a transient performance of the natural gas engine/generator beyond a nominal 10% to 15% (step load) of rated load. Generally, such natural gas engine/generators have a step capability of 50% of rated power. Certain added systems, such as transient richening and turbocharger bypass, enables the natural gas engine/generators to accept 10% transient load step and to reject transient load steps up to 25% of rated power from any given load point.
FIG. 3 is a typical step loading profile (i.e., step load increase or decrease as a percentage of rated kW per time increment) from the measured rig data during pipe tripping operations. Positive percentage is added rig power from one generator and negative percentage is load shedding. This is normalized to a rated power of 1350 kW, at sea level. This data shows a properly set generator control uniquely tuned to avoid large load changes which could lead to an interruption in generator operation and power loss. While these data is typical of tripping events, for drilling, higher transient peaks (especially load shedding of 25% or more) have been noted but with lower average load changes.
FIG. 4 illustrated the typical rig power demand during tripping operations. In particular, FIG. 4 shows the actual gas drilling rig data and data plots highlighting these rig power demands. This plot shows twenty-four hours of data and shows typical cyclical power profiles for tripping and drilling operations. FIG. 5 shows the typical power demand during drilling operations over a twenty-four hour period.
In the past, various patents and patent publications have been issued that relate to power usage and the control of such power usage by drilling rig systems. For example, U.S. Pat. No. 4,590,416, issued on May 20, 1986, to Porche et al., teaches a closed loop power factor control for power supply systems. This power factor controller for alternating current/direct current drilling rigs. The power factor controller utilizes a uniquely controlled, unloaded, over-excited generator to reactive power to maintain the rig's power factor within prescribed limits during peak demand operations. In particular, this method includes the step of: (1) sensing the instantaneous system power factors; (2) comparing the sensed instantaneous power factor to a prescribed power factor; (3) forming a power factor control signal indicative of the difference between the sensed power factor and the prescribed power factor; (4) providing a field excitation signal to an unloaded over-excited generator operated in the motor mode in proportion to the power factor control signal so as to cause the over-excited generator to generate the requisite reactive power to correct the system's power factor to the prescribed power factor; and (5) coupling the output of the over-excited generator to the power system.
U.S. Patent Publication No. 20088/0203734, published on Aug. 28, 2008 to Grimes et al., describes a wellbore rig generator engine power control system. This system controls power load to a rig engine. This system includes a sensor for controlling a rig engine and a sensor for sensing the exhaust temperature of a rig engine. The sensor is in communication with the controller so as so as to provide the controller with signals indicative of the exhaust temperature. The controller maintains power load to the rig engine based on the exhaust temperature.
U.S. Patent Publication No. 2009/0195074, published on Aug. 6, 2009 to Buiel, shows an energy supply and storage system for use in combination with a rig power supply system. This system includes a generator start/stop and a power output control. A bi-directional AC/DC converter converts the AC power generated by the engine-generator. The power supply is adapted to draw energy from the storage system when the rig motor exceeds the capacity of the generator.
U.S. Patent Publication No. 2009/0312885, published on Dec. 17, 2009 to Buiel, teaches a management system for drilling rig power supply and storage. This management system has a power generator coupled to rig loads. The power generator is used for powering and charging the storage system. The energy storage system draws energy from the storage system in periods of high power requirements and distributes excess energy to the storage system in periods of lower power requirements. The output of the power generator is managed based on the rig power usage wherein the output is increased when the rig power requirements are above a preselected threshold and wherein the output is decreased when the rig power requirements fall below a preselected threshold.
U.S. Patent Publication No. 2011/0074165, published on Mar. 31, 2011 to Grimes et al., describes a system for controlling power load to a rig engine of a wellbore rig. The system includes a controller for controlling the rig engine and a sensor for sensing the exhaust temperature of the rig engine. The sensor is in communication with the controller for providing to the controller signals indicative of the exhaust temperature. The controller maintains the power load to the rig engine based on the exhaust temperature.
U.S. Pat. No. 7,311,248, issued on Dec. 15, 2009 to the present inventor, provides a system for managing energy consumption in a heave-compensating drawworks. This system includes a power supply, a winch drum connected to the power supply so as to receive power from the power supply, a flywheel connected to the winch drum and to the power supply, and a controller connected to the power supply and to the winch drum for passing energy to and from the flywheel during an operation of the winch drum. The flywheel includes a disk rotatably coupled to an AC motor. This power supply includes a first pair of AC motors operatively connected on one side of the winch drum and a second pair of AC motors operatively connected on an opposite side of the winch drum.
It is an object of the present invention to provide an energy storage system for use on a drilling rig which allows natural gas engine/generator to operate with the same reliability and responsiveness as that of a diesel engine/generator.
It is another object of the present invention to provide an energy storage system which improves rig efficiency through energy recovery.
It is another object of the present invention to provide an energy storage system which serves to reduce the amount of wasted fuel that had previously been lost in resistive load banks.
It is another object of the present invention to provide an energy storage system which can reduce natural gas fuel consumption and reduce emissions.
It is a further object of the present invention to provide an energy storage system which allows operators to utilize wellhead gas as the fuel for the generator system.
It is still another object of the present invention to provide an energy storage system which serves as an uninterruptible power supply for use during fuel interruptions.
It is still a further object of the present invention to provide an energy storage system which reduces the vulnerability of the generator's output to variations in wellhead gas flow rates and methane contents.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.